The present invention relates generally to optical films, and more specifically to optical films that change color as a function of viewing angle.
The present invention pertains to optical films that are useful in colored displays. Such displays are frequently used as a means to display information in an eye-catching manner, or to draw attention to a specific article on display or for sale. These displays are often used in signage (e.g., outdoor billboards and street signs), in kiosks, and on a wide variety of packaging materials.
It is particularly advantageous if a display can be made to change color as a function of viewing angle. Such displays, known as xe2x80x9ccolor shifting displaysxe2x80x9d, are noticeable even when viewed peripherally, and serve to direct the viewer""s attention to the object on display.
In the past, color has usually been imparted to displays by absorbing inks which are printed onto card stock or onto a transparent or translucent substrate. However, such inks are typically not color shifting (i.e., the colors of such inks do not normally change as a function of viewing angle).
Some color shifting inks have also been developed, chiefly for use in security applications. However, in addition to their considerable expense, some inks of this type are opaque and are therefore not suitable for backlit applications. Furthermore, such inks are typically based on multilayer stacks of isotropic materials, and hence lose color saturation as viewing angle increases.
Color shifting pigments are also known. For example, a family of light interference pigments are commercially available from Flex Products, Inc. under the trade name CHROMAFLAIR(copyright), and these pigments have been used to make decals. The product literature accompanying these decals describes them as consisting of color shifting pigments in a commercial paint formulation, which is then applied to a vinyl substrate. However, the color shifting effect provided by these materials is only observable at fairly large oblique angles, and is limited to a shift between two colors. Also, these materials, which are apparently described in U.S. Pat. No. 5,084,351 (Phillips et al.), U.S. Pat. No. 5,569,535 (Phillips et al.), and U.S. Pat. No. 5,570,847 (Phillips et al.), all assigned to Flex Products, exhibit fairly low color intensity (see, e.g., FIGS. 7-9 of U.S. Pat. No. 5,084,351). Similar materials are described in U.S. Pat. No. 5,437,931 (Tsai et al.).
An iridescent plastic film is currently sold under the trade name BLACK MAGIC(trademark) by the Engelhard Corporation. The film has been advertised in Cosmetics and Personal Care Magazine (September-October 1997) as a black tinted, translucent film 0.7 mil thick but containing more than 100 layers which provides an effect similar to that seen with neon tetra fish, peacock feathers and oil films. The plastic film is a multilayer stack of optically thin films. Thickness variations in the films results in color variations across the area of the film. Although the deviations of the thickness caliper from its average value are not large, they are significant in terms of the color differences in adjacent areas. The various versions of the film are not labeled as a single reflectance color, but instead as dual colored films. For example, the film is commercially available in blue/green and red/green color combinations, among others.
Other color shifting films have also been developed. Some such films are based on multilayer films of metals, metal salts, or other inorganic materials. Thus, U.S. Pat. No. 4,735,869 (Morita) describes titanium dioxide multilayer films which exhibits various combinations of reflection and transmission colors (e.g., green reflection with magenta colored transmission).
Other multilayer color shifting films are known which are polymeric. Thus, U.S. Pat. No. 5,122,905 (Wheatley et al.), in describing the films of U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), notes that the color reflected by those films is dependent on the angle of incidence of light impinging on the film. However, these films are not well suited to color displays, since the color shift observed in these films is very gradual and the color saturation is very poor, particularly at acute angles. There is thus a need in the art for a color shifting film useful in display applications which exhibits sharp color shifts as a function of viewing angle, and which maintains a high degree of color saturation. There is also a need in the art for uniformly colored polymeric interference filters.
Various birefringent optical films have been produced using strain hardening (e.g., semicrystalline or crystalline) materials. These materials have proven advantageous in the production of multilayer optical films, since desired matches and mismatches in the refractive indices of these materials can be achieved through orientation. Such films are described, for example, in WO 96/19347.
There is also a need in the art for a polymeric multilayer optical film having good color uniformity. Multilayer films made from extruded polymeric materials have been found to be highly susceptible to distortions in layer thickness and optical caliper, which result in color variations and impurities across the width of the film. This problem was commented on in Optical Document Security, 251-252 (Ed. R. van Renesse, 1994). In describing the multilayer polymeric films produced to date by Dow Chemical Company and their licensee, Mearl Corporation, the reference notes that control of thickness variations of the individual layers in these films is very difficult and that, as a result, the films exhibit xe2x80x9ccountless narrow streaks of varying color, few of which are wider than 2-3 mm.xe2x80x9d Id. At 251. This problem was also noted in Dow""s patent U.S. Pat. No. 5,217,794 (Schrenk) at Col. 11, Lines 19-32, where it is noted that the processes used to make the films described therein can result in layer thickness variations of 300% or more. At Col. 10, Lines 17-28, the reference notes that it is characteristic of multilayer polymeric bodies having optically thin layers (i.e., layers whose optical thickness is less than about 0.7 micrometers) to exhibit nonuniform streaks and spots of color. A similar comment is made at Col. 2, Lines 18-21, with respect to the films of U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.). As demonstrated by these references, there is a long-standing need in the art for polymeric multilayer optical films (and a method for making the same) which have high color uniformity.
Other polymeric multilayer optical films are known which rely on optically thick or optically very thin layers for their primary reflection band. Such films avoid some of the iridescence problems encountered with other multilayer polymeric films, primarily because the bands of iridescence are too close to be discerned by the human eye. However, since the reflection of visible light is provided by higher order harmonics of primary reflection bands located in the infrared region of the spectrum, the ability of the films to produce high reflectivities of visible light is compromised. There is also a need in the art for multilayer polymeric optical films (and a method for making the same) whose primary reflection bands arise from optically thin layers (e.g., layers having an optical thickness between 0.01 micrometers and 0.45 micrometers) and which exhibit highly uniform color.
These and other needs are met by the color shifting films of the present invention, as hereinafter described.
In one aspect, the present invention pertains to multilayer birefringent color shifting films and other optical bodies having particular relationships between the refractive indices of successive layers for light polarized along mutually orthogonal in-plane axes (the x-axis and the y-axis) and along an axis perpendicular to the in-plane axes (the z-axis). In particular, the differences in refractive indices along the x-, y-, and z-axes (xcex94x, xcex94y, and xcex94z, respectively) are such that the absolute value of xcex94z is less than about one half the larger of the absolute value of xcex94x and the absolute value of xcex94y (e.g., (|xcex94z| less than 0.5k, k=max{|xcex94x|, |xcex94y|}). Films having this property can be made to exhibit transmission spectra in which the widths and intensities of the transmission or reflection peaks (when plotted as a function of frequency, or 1/xcex) for p-polarized light remain substantially constant over a wide range of viewing angles. Also for p-polarized light, the spectral features shift toward the blue region of the spectrum at a higher rate with angle change than the spectral features of isotropic thin film stacks.
In another aspect, the present invention pertains to color shifting films having at least one reflection band. With the proper choice of the numeric signs of the layer birefringences, the z-index mismatch, and the stack f-ratio, either the short or long wavelength bandedges of the reflection bands for s- and p-polarized light are substantially coincident at all angles of incidence. Films of this type, when designed using the bandedge sharpening techniques described herein, exhibit the maximum color purity possible with a thin film stack designed for use over large angle and wavelength ranges. In addition to sharp color transitions and high color purity, such films are advantageous in applications requiring non-polarizing color beamsplitters.
In a further aspect, the present invention pertains to color shifting films having at least one optical stack in which the optical thicknesses of the individual layers change monotonically in one direction (e.g., increasing or decreasing) over a first portion of the stack, and then change monotonically in a different direction or remain constant over at least a second portion of the stack. Color shifting films having stack designs of this type exhibit a sharp bandedge at one or both sides of the reflection band(s), causing the film to exhibit sharp color changes as a function of viewing angle. The resulting film is advantageous in applications such as displays where sharp, eye-catching shifts in color are desirable.
In still another aspect, the present invention pertains to a film in which the main peaks in the transmission spectra are separated by regions of high extinction, and in which the high extinction bands persist at all angles of incidence for p-polarized light, even when immersed in a high index medium. The resulting film exhibits a high degree of color saturation at all angles of incidence.
In yet another aspect, the present invention pertains to a film which reflects near IR radiation with high efficiency, but does not reflect a significant amount of visible light at normal incidence. Such a film may comprise a two material component quarterwave stack, or may comprise three or more materials to make an optical stack that suppresses one or more of the higher order harmonics of the main reflection band or bands, which in turn may be achieved by utilizing an optical repeating unit comprising polymeric layers A, B and C arranged in an order ABCD and by effecting a certain relationship among the refractive indices of these materials. This relationship may be understood by assigning polymeric layer A refractive indices nxa and nya along in-plane axes x and y, respectively, polymeric layer B refractive indices nxb and nyb along in-plane axes x and y, respectively, polymeric layer C refractive indices nxc and nyc along in-plane axes x and y, respectively, and polymeric layers A, B and C refractive indices nza, nzb and nzc, respectively, along a transverse axis z perpendicular to the in-plane axes. The proper relationship is then achieved by requiring nxb to be intermediate nxa and nxc with nxa being larger than nxc (e.g., nxa greater than nxb greater than nxc), and/or by requiring nyb to be intermediate to nya and nyc with nya being larger than nyc (e.g., nya greater than nyb greater than nyc), and by requiring either that at least one of the differences nzaxe2x88x92nzb and nzbxe2x88x92nzc is less than 0 or that both said differences are essentially equal to 0 (e.g., max {(nzaxe2x88x92nzb), (nzbxe2x88x92nzc)}xe2x89xa60). In addition to the above film stack construction, bandedge sharpening techniques may be applied to create a sharp transition from high transmission of visible light to high extinction of the near IR light.
In still another aspect, the present invention pertains to a multilayer color shifting film made from strain hardening materials which exhibits a high degree of color uniformity at a given angle of incidence, and to a method for making the same, wherein at least some of the primary reflection bands in the film arise from an optical stack within the film having optically thin layers (i.e., layers whose optical thickness is within the range 0.01 to 0.45 micrometers). The layers within the optical stack have a high degree of physical and optical caliper uniformity. In accordance with the method of the invention, the distortions in layer thickness and optical caliper encountered in prior art non-strain hardening films is avoided by biaxially stretching the cast web by a factor of 2xc3x972 to 6xc3x976, and preferably, about 4xc3x974, which tends to make the lateral layer thickness variations, and therefore the color variations, much less abrupt. Furthermore, a narrower die can be used in making stretched film compared to making cast film of the same width, and this allows for the possibility of fewer distortions of the layer thickness distribution in the extrusion die because of the significantly less melt flow spreading occurring in the narrower die. Additional control over layer thickness and optical caliper is achieved through the use of a precision casting wheel drive mechanism having a constant rotation speed. The casting wheel is designed and operated such that it is free of vibrations that would otherwise cause web thickness chatter and subsequent layer thickness variations in the down-web direction. It has been found that, absent these controls, the normal vibrations encountered in the extrusion process are sufficient to noticeably affect color uniformity, due in part to the low tensile strength in the molten state of the strain hardening materials that are employed in making the optical films of the present invention. Consequently, the method of the invention has allowed the production, for the first time, of color shifting films made from polymeric materials which have a high degree of color uniformity at a particular viewing angle (e.g., films in which the wavelength values of the bandedges of the spectral bands of light which are transmitted or reflected at a particular angle of incidence vary by less than about 2% over an area of at least 10 cm2. The films resulting from the method exhibit essentially uniform layer thickness and optical caliper within the optical stack, thereby resulting in color shifts that are sharper and more rapid as a function of viewing angle as compared to films having a lower degree of physical and optical caliper uniformity.
In a related aspect, the present invention pertains to color shifting films that are made with strain hardening materials (e.g., strain hardening polyesters). The reflectivity, or extinction, of a reflectance band increases as a function of both the number of layers tuned to that wavelength band and the index differential of the layer pairs. The use of strain hardening materials, which exhibit high indices of refraction after stretching, creates large index differentials when paired with selected low index polymers. The required number of layers decreases in direct proportion with an increase in the index differential. Additionally, the layer thickness uniformity can be improved as the number of layers is decreased, since a lower number of layers lessens the dependence on layer multipliers and large feedblock sizes to produce the required number of layers As a result, polymeric film stacks can be made with more precise control of layer thickness for improved spectral characteristics.
In yet another aspect, the present invention relates to color shifting films that behave as polarizers over one or more regions of the spectrum. Such films exhibit color shifts when viewed in transmission, or when viewed in reflection after being laminated to (or coated with) a white, diffusely reflective background such as cardstock. The color shifting polarizers may also be combined with other polarizers or mirrors to produce a variety of interesting optical effects.
The color shifting films of the present invention may be used advantageously as low absorbence materials in displays, providing bright display colors with high luminous efficiency. The display colors may be readily derived by coupling a source of broadband light to the optical film in such a way that various colors of the source light can be viewed in either transmission or reflection. In certain embodiments, the film may also be combined with a broadband mirror. Thus, for example, when the films are combined with a broadband mirror such that the film and the mirror are approximately parallel but are separated by a small distance, an article is obtained which exhibits 3-D xe2x80x9cdepthxe2x80x9d. The film may be formed into several different geometries and combined with different light sources to advantageously utilize the high spectral reflectivity and angular selectivity of the film.